1. Field of the Invention
The present invention relates to cementing slurries containing particles and fibres. In particular, it relates to such fluids in which the sedimentation of the particles or fibres in the slurry is prevented or hindered. Such slurries find applications in well cementing operations.
2. Description of Related Art
The use of particles or fibres in well treatment fluids such as cements has been previously proposed. One such example is described in European Patent No. 1086057 which describes the use of amorphous cast-iron platelet particles in oil-well cements to provide added toughness and impact resistance. However, there is a large density difference between the particle and the base fluid—the cement slurry with which the particles are mixed—so special care must be taken to prevent the sedimentation of the particles. When the particles are metallic, for instance based on cast iron, the density difference is commonly around 5000 kg/m3. Preventing the sedimentation of these fibres is usually ensured by viscosifying the base fluid. The rheology of the base fluid is characterized by a minimum of two parameters, the high shear rate viscosity and the yield stress, which quantifies the low shear rate viscosity of the fluid. There are many drawbacks to viscosifying the base fluid, for instance increase friction pressure drops when the fluid is pumped through tubulars, difficulties in mixing it and increased cost due to the use of viscosifying agents.
EP 621 247 describes the use of selected particle size distributions for solid materials used in cementing slurries to provide slurries that are stable against settling or sedimentation.
EP 721 050 and U.S. Pat. No. 5,501,275 both describe fluids containing a base fluid, a first fibre or particle component and a second fibre component, and relate to the use of fibres in the fluid to limit the movement of particulate materials in the fluid such as sand or proppant particles